Hello Ash,
I think you mean “without” an estate agent, right?
There are some basic online valuation tools, some free, some with a small fee of €5 – €10 for a valuation report (all in Spanish in think):
http://www.yaencontre.com/tasaciones/inicioTasaciones
http://casatoc.es/tasacion-online-viviendas
http://www.tinsa.es/es/store/stima
https://valoracion.euroval.com/
http://www.stvalora.com/es/tasacion-online.html
If you are looking at a property in a city where the market is quite deep and liquid (like Barcelona or Madrid) you can also get a pretty good idea of value by looking at the portals, finding good comparable asking prices, and then knocking off 10% to 20%.
Let us know how you get on.